The present invention relates to an optoelectronic transmitter having an improved control circuit for driving a semiconductor laser. The transmitter disclosed herein may also be included within an optoelectronic transceiver which incorporates both an optical transmitter and an optical receiver for providing bidirectional data communication. The control circuit of the present invention provides for rapid enabling and disabling of the semiconductor laser such that the optical output power emitted by the laser can be both extinguished and reestablished in a nearly instantaneous manner.
Optoelectronic transmitters are well known in the art. Such devices provide an optical communication link between electronic devices such as computers. In operation, binary voltage signals are generated within a host device and input to the optoelectronic transmitter. The transmitter converts the voltage signals into light pulses which are transmitted over an optical fiber. An optical receiver located at the opposite end of the optical fiber converts the transmitted light pulses back into voltage signals which can be read by the receiver's host device. Earlier generations of optoelectronic transmitters employed LEDs as the active element. More recently, however, semiconductor lasers have begun to replace LEDs due to their improved performance characteristics.
While semiconductor lasers offer a number of performance advantages over LED's, they also raise safety concerns due to the possible adverse effects of laser radiation on the eyes. Therefore, it is necessary to design such transmitters with an effective disabling circuit which blocks the transmission of laser radiation immediately upon the detection of a fault or the receipt of an output disable signal from the host device. In addition to a fast acting disable circuit, it is also desirable to have a control circuit which energizes the output signal of the laser to full power quickly in order to reestablish communication over the data link. This allows the transmitter to be connected to an active host, and begin transmitting data almost immediately without generating bit errors during a long ramp up period as the laser output builds to full power.
Many industry standards and specifications set performance requirements for the disable time and turn on time for optical transmitters. As an example, a Media Interface Adapter (MIA) is an optoelectronic transceiver module for use in the massive data storage industry. The operational requirements of the MIA are defined by an ad hoc committee of optoelectronics manufacturers independent of American National Standards Institute (ANSI) committee X3T11. The MIA specification requires that the transmitting semiconductor laser must disable the optical signal in less than 2 milliseconds after receiving a disable signal or detecting a fault. Similarly, the MIA specification also requires that upon initial power up of the transmitter, the output signal of the semiconductor laser must reach operating power in less than 2 milliseconds.
The average optical power emitted by a semiconductor laser transmitter is defined as the quiescent output power P.sub.Q. In transmitting signals, binary ones are represented by an output power P.sub.Q +.DELTA.P and binary zeroes are represented by an output power P.sub.Q -.DELTA.P. The quiescent output power is established by a DC input current I.sub.Q supplied to the semiconductor laser. The output power of the laser follows the DC bias current, in other words, the greater I.sub.Q supplied to the laser, the greater P.sub.Q output by the laser. Since the output power of the semiconductor laser is directly proportional to the input current, the rise time necessary to increase the output power from zero to full power and the fall time to drop the laser output from full power down to zero, depends on how rapidly the input current to the laser can be supplied and withdrawn.
In addition to concerns about the rise time and fall time of the device, it is also necessary to closely regulate the output power of the device. In order for the receiver located at the opposite end of the fiber optic link to clearly distinguish between ones and zeroes, it is necessary that the power levels for each logic state are consistent and adequately separated. This is only possible if P.sub.Q is maintained within a very limited band. Additionally, due to the varying operating characteristics of individual semiconductor lasers, it is also necessary to calibrate the quiescent output power of individual semiconductor lasers to different levels. This adds an additional level of complexity to the electronic circuitry for driving the semiconductor laser.
Automatic power control is generally accomplished by the addition of a control loop which sets the level of input current supplied to the semiconductor laser based on the amount of optical power being emitted by the laser. In such a control loop, a monitor circuit measures the output power of the laser. The measured output power is compared to a reference, and the input current is adjusted accordingly. If the output power is below the desired level, the input current is increased. If the output power is above the desired level, the input current is decreased. Such a control loop quickly forces the output of the laser to a steady state condition where the output power emitted by the laser does not deviate from the desired quiescent operating power.
FIG. 1 shows an example of a prior art control circuit for a semiconductor laser transmitter. A regulated reference voltage of 2.5V is established by R.sub.1 and zener regulator U.sub.1. The 2.5V reference voltage is further divided between R.sub.2 and R.sub.3 to supply a 1.25V reference to the inverting terminal of operational amplifier U.sub.2. The 2.5V reference voltage supplies the power monitoring circuit as well. The power monitoring circuit comprises R.sub.6 and photodiode diode U.sub.3. U.sub.3 is positioned near the transmitting laser U.sub.4, and is oriented such that a portion of the optical energy emitted by U.sub.4 will strike U.sub.3. The 2.5V reference reverse biases U.sub.3, causing a small reverse current to the flow through U.sub.3 and the monitor circuit generally, including R.sub.6. The magnitude of the reverse current through U.sub.3 is modulated by the amount of optical radiation striking the photodiode U.sub.3. Since this will be proportional to the total amount of optical power emitted by semiconductor laser U.sub.4, the magnitude of the reverse current will be proportional to the emitted optical power as well. Node D is created by the junction of R.sub.6 and U.sub.3. The reverse current through U.sub.3 causes a voltage drop across R.sub.6 such that the voltage at Node D will equal 2.5V minus the voltage drop across R.sub.6. Node D is also connected to the non-inverting input of operational amplifier U.sub.2 such that the voltage at Node D is compared to the voltage present at Node C which is connected to the inverting terminal of U.sub.2. The voltage at Node D represents the feedback monitor voltage of the control loop, and is inversely proportional to the optical output of the laser U.sub.4. As the output power of U.sub.4 increases, the voltage present at Node D decreases. Conversely, from the perspective of the laser, the output power emitted by the laser will follow the voltage at Node D. As voltage at Node D increases relative to the reference voltage at Node C, the output power of the laser will increase in response.
The circuit of FIG. 1 regulates the optical power emitted by laser U.sub.4 by regulating the current flowing through transistor Q.sub.1. Operational amplifier U.sub.2 is configured as a differential integrator, which compares the voltage present at Node D to the 1.25V reference voltage established between resistors R.sub.2 and R.sub.3. The output voltage of U.sub.2 controls the base voltage of transistor Q.sub.1, and the base voltage of Q.sub.1 controls the bias current flowing to the laser U.sub.4. Increasing the base voltage increases the bias current, and likewise, reducing the base voltage reduces the bias current.
The output voltage of U.sub.2, and thus the base voltage of Q.sub.1, is determined by the voltage differential between the inverting and non-inverting terminals of U.sub.2. As the voltage differential increases in the positive direction, indicating that the laser is emitting less power than the desired quiescent power, the output voltage of U.sub.2 increases. Conversely, as the voltage differential increases in the negative direction, indicating an output power greater than the desired quiescent output power, the output voltage of U.sub.2 decreases. This varying output voltage alters the base voltage of Q.sub.1, thereby adjusting the bias current input to the semiconductor laser U.sub.4. The input current is continually adjusted in this manner such that, as more current is supplied to the laser the output power of the laser is increased, and as less current is supplied to the laser, the output power of the laser is decreased.
This control loop quickly forces the output power of the laser into a steady state condition. The current allowed to the flow to the semiconductor laser, and thus the optical output power of the laser, is tightly regulated such that the monitor voltage present at Node D and input to the non-inverting terminal of U.sub.2 will very nearly match the 1.25V reference voltage at the inverting terminal of U.sub.2. The output power emitted by the laser under these steady state conditions represents the quiescent output power of the transmitter.
With the circuit of FIG. 1, the quiescent power level can be calibrated or adjusted in one of two ways. First, the 1.25V reference developed at Node C can be changed to another voltage. This changes the voltage at the inverting input to the operational amplifier U.sub.2 such that the laser U.sub.4 must generate more or less optical power in order to generate a feedback monitor voltage which matches the new reference level. Thus, the control circuit will reach steady state at a different quiescent power level. The reference voltage at Node C can be altered by changing the resistance ratio between resistors R.sub.2 and R.sub.3.
The second method for adjusting the quiescent power is to alter the ratio between the feedback monitor voltage and the optical output power of the laser. This can be accomplished by changing the resistance of R.sub.6 or by changing the voltage supplied to the monitor circuit. By changing the ratio between the output power of laser U.sub.4 and the feedback monitor voltage at Node D, more or less output power must be delivered by U.sub.4 in order to produce a feedback monitor voltage that matches the reference voltage applied to the inverting terminal of U.sub.2. Again, the result is that the control loop will reach steady state conditions with the semiconductor laser emitting a different quiescent power level.
The prior art circuit of FIG. 1 includes provisions for quickly disabling semiconductor U.sub.4. This is accomplished by transistor Q.sub.2 which interrupts the flow of current to transistor Q.sub.1 whenever a +5V disable signal is applied to the base of Q.sub.2. Transistor Q.sub.2 is placed in series with Q.sub.1 and acts as a switch enabling and disabling the current supplied to the semiconductor laser. When a DISABLE signal is received, the +5V signal reverse biases the base/emitter junction of transistor Q.sub.2, effectively blocking current flow through Q.sub.2. Under these conditions, no appreciable current will flow to semiconductor laser U.sub.4, regardless of the base voltage applied to the base of Q.sub.1. When the DISABLE signal is removed, Q.sub.2 again becomes conductive, and Q.sub.1 acts as a current source supplying the bias current to U.sub.4.
Transistor Q.sub.3 is added to the circuit to prevent the bias control loop from running away while the disable condition exists. Without Q.sub.3, if transistor Q.sub.2 is merely switched off while the control loop is controlling the laser, the output power of the laser will drop off immediately. The bias control loop will sense the drop in power, and attempt to compensate for the lost power by supplying more bias current to the laser. Since the actual current to laser U.sub.4 is blocked, U.sub.4 no longer radiates energy, and no optical energy strikes the monitor diode U.sub.3. This reduces the reverse current through the monitor circuit to negligible levels, such that the voltage drop across R.sub.6 is insignificant. While the reference voltage applied to the inverting terminal of U.sub.2 remains at 1.25V, the voltage on the non-inverting input of U.sub.2 is raised to nearly 2.5V. This represents a large positive deviation between the inverting and non-inverting terminals of U.sub.2. This raises the output voltage of U.sub.2, thereby raising the base voltage of transistor Q.sub.1. Under these conditions the base voltage of Q.sub.1 is raised to the point where an extremely large current would flow to semiconductor laser U.sub.4 if the current were available. However, because Q.sub.2 is switched off, no appreciable current can actually flow. This represents a dangerous condition in that, if the disable signal is removed, Q.sub.2 again becomes conductive and a very large current flows to the semiconductor laser before the control loop has time to take control of the bias current. This may result in a short burst of excessive optical power being emitted by the laser. Such an uncontrolled burst of power could create an unsafe condition for personnel in the vicinity of the laser or may damage the semiconductor laser itself.
The circuit of FIG. 1 prevents the runaway condition by incorporating transistor Q.sub.3 into the design. Q.sub.3 acts as a diode in series with the disable circuit. When the disable signal is present, Q.sub.3 passes the +5V DISABLE signal (minus the 0.7V base-emitter voltage drop across Q.sub.3) to the inverting terminal of U.sub.2. This has the effect of raising the reference voltage input to the control loop. Instead of the control loop attempting to stabilize the monitor voltage at 1.25V, the control loop attempts to stabilize the monitor voltage at +4.3V. This requires less optical power from U.sub.4 so that the control loop attempts to deliver less bias current to U.sub.4. Since the maximum voltage applied to the monitor circuit is 2.5V, the monitor voltage will never reach the new +4.3V reference even though no current is supplied to laser U.sub.4. Under these conditions, the output voltage of U.sub.2 will be pulled down toward ground potential. When the transmitter is re-enabled, the control loop will initially supply little or no bias current to the laser U.sub.4. When the disable signal is removed, Q.sub.2 again becomes conductive, and the 1.25V reference is restored to the inverting terminal of U.sub.2. The control loop regains control of the output of the semiconductor laser, and reestablishes steady state conditions at the calibrated quiescent operating level.
The prior art circuit in FIG. 1 is very effective in controlling the output power of a semiconductor laser. The circuit also provides a rapid means of disabling the semiconductor laser. In fact, by properly selecting Q.sub.2, a cut-off time of less than 2 milliseconds is readily achieved. However, a problem with the circuit of FIG. 1 is that the startup time required for the control loop to regain control of the output of laser U.sub.4 and begin transmitting a stable quiescent power signal is much too long. The control loop must be configured to have a low frequency response in order to prevent laser output deviations resulting from transmission of the data signal from affecting the quiescent current supplied by the control loop. This is accomplished by selecting capacitor C.sub.1 to be relatively large, on the order of 0.1 .mu.F. A large C.sub.1, however, greatly extends the rise time necessary for the circuit of FIG. 1 to regain control of the output laser. During the disable period, the output of U.sub.2 is driven toward 0V and the +5V DISABLE signal is connected to the inverting input terminal of U.sub.2. Thus, a large voltage is charged across C.sub.1. When the disable signal is removed, the voltage across C.sub.1 must be discharged through the resistor combination R.sub.4 +R.sub.2 .parallel.R.sub.3 before steady state conditions can be established at the output of U.sub.2. Because C.sub.1 is large, the C.sub.1 (R.sub.4 .parallel.R.sub.3) time constant is much greater than 2 milliseconds, and dissipating the charge across C.sub.1 delays the transmission of data via the laser U.sub.4. Without a more rapid response, a transceiver utilizing this type of control circuit would not be well suited for applications requiring hot plugging where the transceiver must begin transmitting immediately upon being inserted into a system. What is needed is a control circuit for regulating the output power of a semiconductor laser wherein the output power emitted by the semiconductor laser can be disabled and re-enabled each in less than 2 milliseconds. Such a circuit should be capable of controlling the current to the semiconductor laser in a safe manner such that the current is controlled at all times, thereby eliminating the risk of an uncontrolled burst of optical power when the laser is re-enabled.